Flac audio file stands for Free Lossless Audio Codec. These have very little compression unlike MP3's so this makes there file size usually really large.

Flac audio format is an audio compression codec primarily authored by Josh Coalson and Ed Whitney. As its name implies, FLAC employs a lossless data compression algorithm: a digital audio recording compressed by FLAC can be decompressed into an identical copy of the original audio data. Audio sources encoded to FLAC are typically reduced to 50–60% of their original size

If you like your audio to sound 100% then the wav audio format is the best thing, but these are extremely large files so the flac file is the next best thing.

Converting flac to MP3 this can be done with lots of free software out there http://www.brothersoft.com/downloads...converter.html

How to convert flac files in itunes using a third party program called fluke (mac only)

fluke for itunes only mac os http://blowintopieces.com/fluke/
fluke for PC users http://www.dbpoweramp.com/download.htm